Optical and Chemical Monitoring during Foil Penetration Experiments to Study Intergranular Corrosion in AA2024

Abstract

A modified version of the foil penetration technique based on an optical detection of the full penetrating intergranular defects through thin foils was applied to AA2024 samples of various thicknesses in contact with a dilute chloride solution. As the rear side of the foil was free during the experiments, it was straightforward to measure the pH of the electrolyte trapped inside the grain boundary network leaking through the emerging defects on the rear side of the foils. The penetration regime in the longitudinal direction was determined for various exposure and electrochemical conditions confirming the decrease of the propagation kinetics as function of the depth of the intergranular defects. pH measurements were used to support mass transport simulation inside a pore electrode mimicking the propagation of the anodic head of grain boundaries. After validation under potentiostatic conditions, the same modelling approach was applied under free corrosion conditions, to discuss the location of cathodic areas during immersion and droplet exposures. Frankel and coworkers 1 considered the anisotropy of the intergranular corrosion process and performed foil penetration tests using various electrochemical conditions. They proposed that the propagation rate of intergranular corrosion followed a power law of the following type:where d is either the nominal thickness of the full penetrated foil or the resulting IG path length, a and n are constants (n is between 0 and 1) and t the time for the full penetration. To discuss these results in terms of IG propagation rate the raw data can be inverted and expressed as the depth of the fastest growing site as a function of time. However the physical meaning of the exponent and its value as function the nature of the alloy have not been discussed to our knowledge. Nevertheless, interesting work described in Ref. 2 illustrated application of such a law (relation 1) deduced from laboratory experiments to operational aircraft. In these practical cases there was a large variation in corrosion growth rates from one section of a wing to the next or from an aircraft to the next. But it was demonstrated that it was possible to provide a distribution for coefficients of relation (1), especially for coefficient a, which could be deduced from documented corrosion data collected, for example, from repair data.Relation (1) which means that the intergranular (IG) propagation rate on aluminum alloys (AA2024) is decreasing as function of the IG path length has been validated under potentiostatic conditions. 1 Moreover this trend has been also frequently discussed in other studies under free corrosion conditions. 3,4 Nevertheless, the origin of this feature has not been largely discussed quantitatively, especially as the inner surface of the "IG crevice" behind the active anodic head at the point of attack 5 can also suffer dissolution at a reduced rate and at long exposures as illustrated in different works showing a blunting of the IG path.6,7 Consequently the mass transport and/or the ohmic dr...

Thin Foil Penetration Technique at a Local Scale: Towards an Analysis of the Current Transients during Intergranular Propagation on AA2024 Alloy

“…As schematized in Figure 2, an optical detection system (digital optical microscope Keyence VHX 5000) was positioned in front of the droplet cell to detect the emergence of full penetrating IG defects in a similar way than in large scale experiments. 6,7 As it will be illustrated in the discussion of this paper, the results obtained by digital optical microscopy improved the optical resolution compared to preliminary results 8 and allowed the successive steps of the detectable events corresponding to the full penetration of selected grain boundaries to be distinguished.…”

“…This very reproducible grooving was used as a "precorrosion" procedure to control the initiation of grain boundary dissolution which led to the improvement of the accuracy of the measurement of the IG propagation rate. 7 To support the beneficial effect of switching the potential to a value lower than the breakdown potential (−580mV(SCE)), potential steps at −480mV(SCE) were followed by very long maintains at −600 mV(SCE). As shown in Figure 3b (II), initial short "fissures" initiated at −480mV(SCE) widened after long exposures whereas the alloy remained still sensitive to IGC at −600mV(SCE) but at a very reduced rate as illustrated by the thin filaments ahead.…”

“…Local scale experiments.-The droplet cell set up was combined with the optical detection of reduced number of full emerging defects to perform modified thin foil penetration experiments compared to previous large scale conditions. 6,7 This was achieved by recording the current response on a polished thin foil (60 μm-thick) imposing the same potential step at −480mV (SCE) as previously.…”

“…The transient current response and optical detection of the emerging defects were recorded in real time. As explained in a previous publication, 7 continuous imaging of the rear side of the specimen allowed "cracks" induced by the emergence of any full penetrating IG paths after 60 μm in depth propagation to be located. It was validated that there is not a large risk of branching of the IG path (Figure 3bI) at short times.…”

A Review of the Application of Finite Element Method (FEM) to Localized Corrosion Modeling

“…These new codes allow the corrosion researcher to focus on defining the problem and the boundary conditions rather than searching for the best partial differential equation solver. 44,46,[51][52][53][54][55][56][57][58][59][60][61][62][63][64]…”